For a wheel of a four-wheeled motor vehicle (hereinafter, as appropriate, referred to as a vehicle in short) such as a passenger automobile, certain “alignment angles” including a camber angle (an angle formed by a longitudinal center line of the wheel and a plumb line when the wheel mounted on the vehicle is viewed from a front face of the vehicle), a toe angle (an angle formed by a horizontal diameter line (a equatorial line) of the wheel, and a straight line passing the anteroposterior center of the vehicle when the vehicle travels straight forward), and a caster angle (an angle formed by a king pin shaft and a plumb line when the wheel is viewed from a side face of the vehicle) are generally determined in consideration of: obtainment of traveling stability (for example, steerability and stability); and prevention of local abrasion of a pneumatic tire mounted on a vehicle.
A specified value of those alignment angles is determined for each vehicle in a designing stage of vehicles, and the alignment angles of each wheel are adjusted, as needed, based on the specified value by use of a wheel alignment angle measuring apparatus (what is termed a wheel alignment measuring apparatus) , for example, in replacing a pneumatic tire or a tire wheel.
Additionally, a method of adjusting an alignment angle (toe angle), which should be specified, is disclosed in consideration of a possibility that an ideal value of the alignment angles may change with a change of vehicle conditions such as a case where pneumatic tires or tire wheels mounted on a vehicle are exchanged (for example, Japanese Patent Application Laid-open Publication No. 2000-62639 (pp. 4-7 and FIG. 15)) In this method, the alignment angle (toe angle) is adjusted according to a characteristic of a lateral force Fy generated in a wheel, more specifically, a variation rate of the lateral force Fy (for example, the sum of squares of a first-order differential value).
In this adjusting method, first of all, the lateral force Fy is continuously measured in a predetermined period including a time point when a reference wheel (for example, a rear wheel) runs on a predetermined protrusion (for example, a plate-like object). Subsequently, based on a waveform indicating temporal changes of the lateral force Fy generated in the rear wheel, a toe angle of a non-reference wheel (for example, a front wheel) generating a lateral force Fy forming a waveform approximated as much as possible is obtained through an arithmetic operation.
According to the adjustment method, a variation rate (a characteristic) of the lateral forces Fy generated respectively in the rear wheel and in the front wheel become equal to each other, whereby a steering characteristic of a vehicle can be made close to the neutral steer.
However, the above described method of adjusting an alignment angle (a toe angle) has had a problem that should be solved for improvement. In other words, in the above described method of adjusting an alignment angle (a toe angle), the toe angle of the front wheel generating the lateral force Fy forming the waveform approximated as much as possible to the foregoing waveform is obtained through the arithmetic operation based on the waveform indicating the temporal changes of the lateral force Fy generated in the rear wheel, but there has been a limit in adjusting the toe angle so as to generate the lateral force Fy forming the waveform approximated as much as possible to the foregoing waveform.
That is, there has been a problem that, depending on a kind or the like of a vehicle, a pneumatic tire or a tire wheel, there is a case where temporal changes of the lateral force Fy generated in the front wheel cannot be sufficiently approximated to the temporal changes of the lateral force Fy generated in the rear wheel.
Consequently, the present invention has been made in consideration of a situation as described above, and an object of the present invention is to provide a wheel alignment angle measuring apparatus and a wheel alignment angle measuring method, which can determine an alignment angle enabling characteristics of lateral force generated respectively in a front wheel and in a rear wheel to be approximated to each other.